1. Field of the Invention
The present invention relates to an organic light-emitting diode (OLED) device, and more particularly to an OLED device comprising a plurality of white light emitting layers with complementary spectra so as to enhance the color rendering of the emitted white light.
2. Description of the Prior Art
An organic electro-luminescence display (Organic EL Display), also known as an organic light emitting diode (OLED), was invented by C. W. Tang and S. A. VanSlyk et al. of Eastman Kodak Company in 1987 and manufactured by a vacuum evaporation method. A hole transporting material and an electron counterpart are respectively deposited on a transparent indium tin oxide (abbreviated as ITO) glass, and then a metal electrode is vapor-deposited thereon to form the self-luminescent OLED apparatus. Due to high brightness, fast response speed, light weight, compactness, true color, no difference in viewing angles, no need of liquid crystal display (LCD) type backlight plates as well as a saving in light sources and low power consumption, it has become a new generation display.
Referring to FIG. 1, a cross-sectional diagram of a conventional structure of an OLED apparatus is illustrated. The conventional OLED apparatus A sequentially includes, from the bottom to the top, a transparent substrate A1, a transparent anode (indium tin oxide, ITO) A2, a hole transporting layer (HTL) A3, an organic emissive layer (EL) A4, an electron transporting layer (ETL) A5, an electron injection layer (EIL) A6, and a metal cathode A7. When a forward bias is applied, holes are injected from the anode A2 and electrons are injected from the cathode A7. Due to the potential difference resulted from the external electrical field, the electrons and holes move in the thin film and hence recombine in the organic emissive layer A4. A part of the energy released by the recombination of the electron and hole pairs excites the emissive molecules from a ground-state to an excited-state in the organic emissive layer A4. As the emissive molecules fall back form the excited-state to the ground state, a certain portion of the energy is released to emit light.
A color rendering index (CRI) is an important light-emitting efficiency index of the OLED device. The CRI means the relative difference between the revealed colors of an object illuminated by an artificial light source and the revealed colors of the object illuminated by a sunbeam. The lower value of the CRI, the more differences between the above revealed colors, and the colors presented by the artificial light source lose more fidelity. In contrast, the higher value of the CRI, the better color rendering of the artificial light source, that it, the colors presented by the artificial light source lose are close to those presented by the sunbeam.
In order to increase the color rendering of the OLED device, most conventional techniques combine a plurality of OLED devices and regulate some parameters, such as color temperature and wavelength, to achieve the effect of high color rendering. However, due to the foregoing conventional technique has to combine several kinds of OLED devices emitting different colors to achieve the effect of high color rendering, then it not only consumes more production cost but also increases the dimension of the products.
In view of this, it is necessary to provide an improved OLED device which can achieve the effect of high color rendering by employing fewer components and saving production costs.